1. Field of the Invention
The present invention relates to an antenna device, and in particular to an antenna device adapted for portable radio apparatus.
2. Description of the Related Art
A portable radio apparatus such as a mobile phone often has a limited mounting space, and thus may suffer from a problem of interference caused by electromagnetic or electrostatic capacitive couplings among an antenna and each of portions of an electrical circuit of the radio apparatus. In particular, the antenna may often face a problem of degraded radiation efficiency.
To the above problems, possible solutions using magnetic material have been proposed. For instance, a conventional portable radio apparatus is disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2001-156484.
More specifically, the radio apparatus disclosed in JP 2001-156484 includes a printed circuit board, a shield case for shielding a portion of the printed circuit board, and an antenna configured to be pulled out of the shield case and to be extended.
The radio apparatus disclosed in JP 2001-156484 may improve a shielding effect, for one thing, by strengthening electrical connections between the shield case and a ground pattern of the printed circuit board in a direction perpendicular to a direction of a radio frequency current induced on the shield case.
The radio apparatus disclosed in JP 2001-156484 may improve the shielding effect, for another thing, by layering magnetic films having an easy magnetization axis in the direction of the radio frequency current induced on the shield case so as to raise a coefficient of reflection of radio waves.
Another example of the possible solutions is a conventional antenna device adapted for a communication apparatus disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2006-222873.
More specifically, the antenna device disclosed in JP 2006-222873 includes a dipole antenna (a feed element) and a parasitic element such as a conductor plate. The antenna device disclosed in JP 2006-222873 may improve impedance matching and a wavelength shortening effect for downsizing by forming the parasitic element from magnetic material or a metal plate with a surface layered by magnetic material, and by controlling parameters of the magnetic material (relative magnetic permeability, relative dielectric constant and a depth) properly.
The radio apparatus disclosed in JP 2001-156484 has an extendable antenna, and is configured to prevent a radio frequency current from being conducted into the portion of the printed circuit board shielded by the shield case by lowering impedance of the shield case so that the radio frequency current may easily flow on the shield case.
The configuration of the radio apparatus disclosed in JP 2001-156484 may hardly be applied to a radio apparatus including a built-in antenna, as, e.g., a positional relationship between the built-in antenna and a printed circuit board is different from a positional relationship between the extendable antenna and the printed circuit board of the radio apparatus disclosed in JP 2001-156484.
The configuration of the radio apparatus disclosed in JP 2001-156484 may hardly be applied in a case where it is difficult to define a direction of the easy magnetization axis uniquely, as the magnetic films may not be layered until the direction of the easy magnetization axis is defined.
The above disclosure of the antenna device in JP 2006-222873 gives an embodiment of the antenna device including magnetic material having relative magnetic permeability of around 10, and refers to neither isotropy/anisotropy of the magnetic material, nor possibility of further improvement of antenna characteristics of radio apparatus by using anisotropic magnetic material of higher relative magnetic permeability.